The present invention relates to a method for evaluating performance of a chemical filter, including ion exchangers, for cleaning a gas. The chemical filter has recently been used in clean rooms in the microelectronics industry such as semiconductor industry or pharmaceutical industry. Namely, the present invention relates to performance evaluation of air-cleaning chemical filter, which can provide information about the time for replacing a chemical filter, such as consumed ion-exchange capacity or residual exchange capacity of the filter material, or which can assess whether or not the analytical level of a component to be removed at the inlet and outlet of a filter is correct without destroying the filter. The present invention is also applicable to quality control during the manufacture process of ion exchangers which constitute air-cleaning chemical filter.
Known chemical filter for removing gaseous components in the air includes activated carbon particles or activated carbon fiber on which an acid or alkali is optionally deposited. Filters comprising an oxide or a metal supported on other carriers are also known.
In recent years, chemical filters using ion exchangers have begun to be used particularly in the semiconductor-related industry, because they can efficiently remove gaseous components at ppb levels without releasing adsorbates.
Ion exchangers are often used in chemical filters in the form of a non-woven or woven fabric because of the high removal efficiency, light weight and formability. Ion-exchange groups include cation-exchange groups such as sulfuric acid and carboxyl groups, as well as anion-exchange groups such as quaternary ammonium and tertiary amino groups. The mechanism by which these ion-exchange groups remove gaseous components is mainly based on a neutralization reaction. Thus, the ion-exchange capacity gradually decreases as a filter using such an ion-exchange group continues to be used. As the consumed ion-exchange capacity increases, the removal efficiency for gaseous components declines, and therefore, the filter must be replaced.
The time for replacing a filter for removing microparticle such as HEPA filter or ULPA filter can be known from pressure loss. However, it is difficult to estimate the time for replacing a chemical filter. Typically, concentrations of gaseous components of interest are determined in an upstream and a downstream of the chemical filter, and a decline in removal rate thereof is an indicator for the replacement.
However, extended suction with an impinger and careful analysis of the absorbed liquid are needed to determine an extremely low gas level in an environment which requires a chemical filter. This involves the possibility that gaseous components to be removed may not be removed by the chemical filter but continue to flow out for a long period and contaminate the surface of semiconductor products before the results come out. In addition, it is very difficult to sample trace gas components, and many samplings are necessary to know if a sampling site is typical of filter performance even if the analytical value is correct. Such an analysis is laborious and time-consuming.
The lifetime of ion exchangers can relatively easily be calculated from concentrations of gas components in a gas, since they are chemical adsorbents in contrast to physical adsorbents such as activated carbon. Thus, the gas level may be determined as described above when the lifetime approaches its end, but the gas concentrations at the inlet is seldom constant. Therefore, the gas level must be regularly analyzed until the removal efficiency declines. This can not solve the above problems. An alternative solution is to replace any filter used for some period even if it still has a remaining ion-exchange capacity, i.e. the capacity to remove gaseous components.
Such a method not only go against the trend to save resources and energy, but also raises costs. Under the present situation, there have been eager demands for a method which can directly and rapidly indicate the time for replacing a chemical filter without destroying or contacting it, and which can eliminate the complexity of the prior art involving indirect determination of the time for replacing a filter by analyzing the level of a trace gas component in the air upstream and downstream of the filter.
Further, it was difficult to determine when and which filter should be replaced in case of large area air-cleaning which usually requires a plurality of filters, because a constant flow can not always be obtained in such a case or if it could be obtained, the gaseous component level is not always constant so that the consumed ion-exchange capacity varies for each filter.